The invention relates to a centrifugal pump with an axial diffusor for a treating liquid, and more particularly to an improvement in an axial diffusor with a plurality of blades having such an incident angle as to prevent revolving and stalling to occur at an inlet portion of the diffusor.
The structure of a conventional centrifugal pump with the axial diffusor will be described with reference to FIG. 1.
In general, the centrifugal pump is provided with an impeller 10, an axial diffusor 12 with a plurality of blades 16. The impeller 10 is engaged with a rotation shaft 22 through a key 11 so as to rotate around a rotation axis together with the rotation shaft 22 where a treating liquid 14 exhibits an increase in its flow rate. In contrast, the axial diffusor is so secured through a slide bearing 19 to the rotation shaft 22 that the axial diffusor 12 with the plurality of blades 16 does not rotate around the rotation axis. Each of the blades 16 is placed at its one side on a side portion of the axial diffusor. Each of the blades 16 is placed at its opposite side on an inner wall of a casing 20. The treating liquid 14 is subjected to a flow straightening by the blades 16 of the axial diffusor 12 where the treating liquid exhibits a reduction of the flow rate and an increase in its pressure.
Such axial diffusor pump suffers from the following undesirable phenomenon concerning the flow of the treating liquid. When the treating liquid flows through the blade 16 of the axial diffusor 12 and thus is subjected to the flow rate reduction and the pressure rise, the following undesirable phenomenon occurs. When the discharge flow rate is in a high flow rate range, the effects of the flow rate reduction and the pressure rise are normally exhibited by the blades 16 of the axial diffusor 12. However, when the discharge flow rate is in a low flow rate range, the above normal effects of the flow rate reduction and the pressure rise are not exhibited. In replacement of the normal phenomenon, the treating liquid exhibits a revolving and a stall at the inlet portion of the axial diffusor 12. Such revolving and stall phenomenon causes a violent axial vibration which reduces the life-expectancy of the slide bearing 19. FIG. 2 illustrates a head-discharge curve. Further, this makes the flow rate and head property inferior thereby lowering the pump efficiency. The revolving and the stall at the inlet portion of the axial diffusor 12 also cause an increase likelihood of hunting. Actually, the available operational flow rate range of the pump is restricted by a generation of the revolving and the stall of the treating liquid.